E-mail used to be a file. A file called something like /var/spool/mail/[username], and new email would appear by text being appended to that file. The idea was that the system could send notifications to you by appending messages there, and you could send messages to other users by appending text to the files belonging to other users, using the program mail.
Later on you could send email to other machines on the network, by addressing it with user name, and @ sign and the name of the computer. I am not 100% sure, and I am too lazy to look it up, but the way this communication happened was using SMTP, simple mail transfer protocol. You’ll notice that SMTP only lets you send mail (and implicitly appending it to the file belonging to the user you are sending to).
Much later Post Office Protocol was invented, so that you could fetch your email from your computer at work and download to your Eudora email client on your Windows machine at home. It would just fetch the email from your file, optionally removing the email from that file as doing so.
As Lotus and Microsoft built groupware solutions loosely built on top of email, people wanted to access the email on the server rather than always download thenm, and have the emails organised in folders, which led to the introduction of IMAP.
Why am I mentioning this? Well, you may -if you are using a UNIX operating system still see the notification “You have mail” as you open a new terminal. It is not as exciting as you may think, it is probably a guy called cron that’s emailing you, but still – the mailbox is the void in which the system screams when it wants your help, so it would be nice to wire it into your mainstream email reader.
Because I am running Outlook to handle my personal email on my computer, I had to hack together a python script that does this work. It seems if I was using Thunderbird I could still have UNIX mail access, but.. it’s not worth it.